The present invention relates to a superconducting wire and a method of producing the same. More particularly, the invention is concerned with a superconducting wire containing a paramagnetic salt as a third component and a method of producing such a superconducting wire.
Recently, it has been proposed to use a superconducting magnet in nuclear fusion systems of magnetic confinement type, as well as in high-energy accelerators. The superconducting magnet, when used in a nuclear fusion system, operates in the presence of a plasma, so that the following problem is encountered due to mutual interaction between the superconducting magnet and the plasma. Namely, when the plasma is started, the superconducting coil is subjected to an abrupt change in the magnetic field from an ohmic heating coil. This change in the magnetic field is on the order of 10 T/S to 50 T/S. The superconducting coil receives also a fluctuating magnetic field from a turbulency heating coil, ranging between about 5000 T/S and 10000 T/S. Furthermore, a drastic change in magnetic field on the order of several hundreds to several thousands T/S is applied to the superconducting magnet when the plasma is abruptly disrupted. These drastic changes in the magnetic field cause an a.c. (alternating current) loss in the superconducting coil to generate heat which heats up the superconducting magnet excessively. The superconducting magnet is heated also by nuclear heat of the particles produced as a result of the burning in the nuclear fusion reactor. As a result of the heating, the superconducting coil is liable to be changed into the normal state.
The same problem is encountered also in a high-energy accelerator incorporating a superconducting magnet. Namely, in such an accelerator, the superconducting magnet may be turned into the normal state by particles coming out of orbits and impinging upon the magnet. Needless to say, countermeasures are taken to prevent the heating of the superconducting magnet by electromagnetically shielding the same from the fluctuating magnetic field and also from the nuclear heat and impinging particles by means of a suitable shielding. These measures, however, cannot satisfactorily shield the superconducting magnet from the fluctuating magnetic field and the particles. It is, therefore, necessary to stabilize the superconducting wire itself, in order to prevent the superconducting magnet from becoming the normal state.
A typical conventional superconducting wire is composed of a region of normal-conducting material such as aluminum, copper or the like and a region of a superconducting material such as niobium-titanium (NbTi) or niobium 3-tin (Nb.sub.3 S.sub.n). The region of the normal-conducting material occupies a cross-sectional area which is several to several tens times as large as that of the region of the superconducting material. Therefore, the heat capacity of the superconducting wire depends almost perfectly on the specific heat and the volume of the normal-conducting material.
FIGS. 1a and 1b show conventional superconducting wires having a circular cross-section and a rectangular cross-section, respectively. In either one of these conventional superconducting wires, a region 2 of the superconducting material is surrounded and covered by a region 3 of the normal-conducting material.
More specifically, each of the superconducting wires shown in FIGS. 1a and 1b has the region 2 of a superconducting material composed of niobium-titanium filaments 2a embedded in a copper matrix 2b and the region 3 of a normal-conducting material which is copper. Assuming that the weight ratio of the superconducting material 2 to the normal conduction material 3 is selected to be 1:20, that the temperature of refrigerant which is usually liquid helium is 4.2K, and that the critical temperature of the niobium-titanium is 10K, the superconducting wire 1 has a heat capacity of 3 mJ/g. Thus, an instantaneous and adiabatic heat input of this amount of heat causes the superconducting wire 1 to be changed into the normal state. Unfortunately, the heat input to the superconducting wire 1 by the aforementioned phenomena often exceeds the above-mentioned amount of heat, although it depends on various factors such as the condition of operation of the reactor and the scale of the reactor itself. This imposes a bottleneck on the application of the superconducting coil to nuclear fusion reactors.
In order to obviate this problem, there have been proposed superconducting wires as shown in FIGS. 2a and 2b which correspond to FIGS. 1a and 1b, respectively. The superconducting wires 1 shown in FIGS. 2a and 2b have the intermediate region 4 of a paramagnetic salt as a component thereof. More specifically, the superconducting wire shown in FIG. 2a and having a circular cross-section is provided with the intermediate region 4 of a paramagnetic salt interposed between the region 2 of the superconducting material and the region 3 of the normal-conducting material, while the superconducting wire 1 shown in FIG. 2b and having a rectangular cross-section is provided with the intermediate regions 4 of the paramagnetic salt interposed between the region 2 of the superconducting material and the region 3 of the normal-conducting material. The paramagnetic salt is an electric insulator and is not conductive thermally. Typical examples of the paramagnetic salt are Ho.sub.2 Ti.sub.2 O.sub.7, MnNH.sub.4 Tutton salt, FeNH.sub.4 alum and CrK alum. The MnNH.sub.4 Tutton salt, FeNH.sub.4 alum and CrK alum exhibit specific heats which are several hundreds to several thousands times as large as the specific heat of the copper at cryogenic temperatures below 1K. These three paramagnetic salts, therefore, are used as materials for use in the method of adiabatic demagnetization.
The superconducting coil is usually used at temperature of 4.2K and, in some cases, at temperature of 1.8K. When HO.sub.2 Ti.sub.2 O.sub.7 mentioned before is used at such low temperature, it exhibits a specific heat which is about 100 times as large as that of the copper and, hence, improves the heat capacity of the superconducting wire considerably. Generally, however, the paramagnetic salt exists in the form of inorganic crystals having large moisture absorbability and high fragility. Therefore, it is quite difficult to shape the paramagnetic salt into wire form together with the metals. Particularly, when the intermediate region 4 of the paramagnetic salt is positioned at the inner portion of the superconducting wire as shown in FIGS. 2a and 2b, the drawing for producing such superconducting wires encounters a difficulty in maintaining the uniformity of the cross-section, often resulting in rupture of the wire during drawing or fluctuation of the superconducting characteristics. Thus, it has been quite difficult to put the superconducting wire having paramagnetic salt into practical use.